I spar this guy sometimes who trys to emulate a bit of a PBF style , he turns very sideways and has his right hand up , left hand down or across the waist

He is orthadox so his left side is very exposed to me, o keep hitting him on the back because he only has to turn a tiny bit and his back is exposed to my field of view

Is it legal for me to hit him there?

Hope you can imagine what situation I am describing, I know it is not the best description!